Various packaging configurations in the semiconductor industry include “surface mounted” packages attached to a printed circuit board (PCB). The packages are both physically and electrically attached to the PCB, with an appropriate type of solder (solder paste) utilized to provide the physical attachment while also providing an electrical connection path between the packages and bond pad locations on the PCB. Some of the packages have leads that extend beyond the outer perimeter of the package housing, while others include electrical contacts formed on a lower package surface (the latter type mating with “solder bumps” on the PCB surface).
When soldering a package onto a PCB, a soldering flux is often mixed with the solder in order to reduce the creation of oxides on either the leads or bond pads. Once the soldering process is completed, corrosive residues of the soldering flux typically remain in the gaps between the top surface of the PCB and the lower surface of the package housing. While the structure is subjected to a cleaning process before performing a next type of encapsulation, the gap between the surfaces may be so small (e.g., less than a millimeter or so) that some of the flux residue cannot be dislodged. As a result, this flux residue becomes embedded within final encapsulated product.
Unfortunately, this embedded flux residue can accelerate the failure of the circuitry by virtue of electrochemical migration (ECM). ECM can be defined in this situation as a movement of metal ions (in the flux residue) through the PCB under the influence of an applied DC bias voltage. This movement produces a dendrite type of morphology; structures such as “tree-like” or “feather-like” filaments that grow through the flux residues between two adjacent metal stripes (or opposing electrodes) as a function of the applied voltage. The higher the applied voltage (i.e., electrical potential differential), the faster the filaments form and grow. If severe enough, ECM leads to current leakage and intermittent shorts within the circuit structure and, consequently, increases the chances for catastrophic failure.
While high pressure deionized water provides the safest means of PCB cleaning, the extremely low clearances between the PCB and mounted packages cannot always ensure that all flux residue has been eliminated.